A rapid, inexpensive and highly reproducible procedure for the detection and isolation of plasmid DNA from virtually any species of Streptococcus was developed and used to screen 124 tetracycline resistant streptococci, from the oral cavity of periodontal patients, for the presence of plasmid DNA. Plasmids were detected in only 21% of these isolates, suggesting that tetracycline resistance among the oral streptococci is, for the most part, not plasmid-mediated. Using the same procedure, together with standard plasmid curing techniques, data were obtained which suggested that among strains of the group N streptococcus, S. lactis, plasmids play a role in the metabolism of at least six different carbohydrates. The new isolation procedure was also used in the examination of the molecular properties of a deleted antibiotic resistance plasmid in a strain of S. anginosus. The transmissible R factor, pAM beta 1, was shown to have lost approximately 50% of its DNA. The resultant molecule was no longer transmissible by conjugation, but still mediated resistance to erythromycin and lincomycin. In addition, the deleted plasmid existed in two unusual molecular configurations.